Field of the Invention
The present invention relates to methods, especially to methods of making lead-carbon couplings such that the interfaces of the carbon materials and the lead materials are joined together with high electrochemical and mechanical stability. These lead-carbon coupling methods are used in forming lead-carbon electrode sheets which are further applied to form electrode sheets of lead-carbon batteries by lead welding.
Descriptions of Related Art
Power storage plays a key role in power management and broader use of renewable energy. In general, power can be stored either by the physical or the electrochemical methods. The electrochemical batteries have become a priority in micro-grid energy storage because they meet requirements of quick charge/discharge and high storage capacity.
For the electrochemical methods, the hybrid lead-carbon battery system formed by electronically connecting in parallel a conventional lead-acid battery and an asymmetric electrochemical supercapacitor provides a viable and economical method for power storage. The supercapacitor acting as a buffer in fast charging/discharging conditions can inhibit the sulfation of the negative electrode (lead plate) of the battery during high rate partial state of charge (HRPSoC) process. By extending the cycle life of the batteries, the cost for each charge/discharge cycle is significantly reduced.
In conventional lead acid batteries, the sulfation occurs when non-conductive lead sulfate (PbSO4) crystals are deposited on the surfaces of the negative electrode during discharge. For deep discharge conditions or during the HRPSoC processes, the sizes of the non-conductive lead sulfate crystals would increase. As the active surface areas of the lead, plate is reduced, the storage capacity and the cycle life of the batteries decrease
One method to reduce the sulfation is to add porous carbon materials on the lead electrode. However, because the carbon materials are very chemically inert, it is very difficult to form a stable lead-carbon interface. Moreover, the porous carbon materials are mostly available in powders which can only be sintered under high pressure (about 400 MPa) and high temperature (about 950° C.) conditions. The lack of a convenient method to form stable lead-carbon couplings limits the maximum amount of the added carbon materials to ˜5 wt %.
Another method to reduce the sulfation is to use the bi-electrode approach in the internal parallel hybrid systems. In this approach, the negative electrode consists of two plates, one being a conventional lead plate and the other a porous carbon plate with high specific surface areas. The porous carbon plate acts as a capacitor to absorb high currents on the bi-electrode system during change and discharge, consequently reducing the current experienced by the lead plate and thus minimizing the sulfation.
Although the bi-electrode approach in the internal hybrid system that combines the conventional lead-acid batteries and asymmetric supercapacitors in one battery cell provides a method for low-cost power storage, the lead-carbon interface at the junctions of the carbon and the lead plate is easily subjected to corrosion because the carbon and the lead are only joined physically. Without forming chemical bonds, the lead at the lead-carbon interfacial region can be easily attacked by the bisulfate ion [HSO4−] to form the electronically non-conductive lead sulfate during discharge. With the accumulation of the non-conductive lead sulfate, the electronic connections at the lead-carbon interface become unreliable and eventually fail. The increase of the contact resistance gives rise to decreased power storage efficiency and low cycle life.
To enhance the electrochemical stability of the lead-carbon interface, various surface modification methods on carbon materials had been used. In some chemical impregnation methods, oxygen functional groups are formed as the anchoring sites for the metal precursors. In some bridging methods, noble metal catalysts, such as Pt and Pd, are used as a bridge to connect the carbon and the lead. In another method, diamond-like carbon layers are grown on the carbon surfaces as a buffer layer to enhance the wetting of the lead on carbon.
Among the many demands for making an electrochemically stable lead-carbon interface, demand for more efficient and lower cost fabrication methods has become increasing popular. Accordingly, there is room for improvement and a need to provide a simple and facile method to form chemical bonding between the lead and the carbon without using the expensive noble metals, or complex chemical pretreatment procedures, or complicated vacuum deposition process. The method can be applied for the fabrication of an electrochemically stable lead contact on the carbon electrode sheets. Such carbon electrode sheets made with a stable lead contact can further be applied for the fabrication of hybrid lead-carbon batteries by directly connecting together the lead contacts on the negative plate in the conventional lead acid battery cell and that on the carbon electrode sheets. This provides a low cost method for mass production of the hybrid lead-carbon batteries.